1. Field of the Invention
This invention relates to a gas stream classifier (an air classifier) for classifying a powder by utilizing Coanda effect, and a process for producing a toner for developing electrostatic images, by means of such a classifier. More particularly, the present invention relates to a gas stream classifier for classifying a powder into particles with given particle sizes while carrying the powder on gas streams and also utilizing Coanda effect and the differences in inertia force and centrifugal force according to the particle size of each particle of the powder so that a powder containing 50% by number or more of particles with a weight average particle diameter of 20 .mu.m or smaller can be classified in a good efficiency, and also relates to a process for producing toners by the use of such a classifier.
2. Related Background Art
For classifying powders, various types of gas stream classifiers are proposed. Among them, there are classifiers making use of rotating blades and classifiers having no moving part. The classifiers having no moving part include fixed-wall centrifugal classifiers and inertial classifiers. As classifiers utilizing inertia force, Elbow Jet classifiers disclosed in Okuda S. "Classification of Ultra-fine Powder", 17 Lecture and Discussion concerning Powder Engineering at Doshisha University, pp. 22, 24 and 27 (1983) and commercially available as products by Nittetsu Kogyo, and classifiers disclosed, e.g., in Okuda, S. and Yasukuni, J., Proc. of International Symposium on Powder Technology '81, 771 (1981) have been proposed as inertial classifiers that can carry out classification of powders having small particle diameters.
In such gas stream classifiers, as shown in FIGS. 15 and 16, a feed powder is jetted into the classification zone of a classifying chamber 32 at a high velocity together with a gas stream, from a feed supply nozzle 16 having an orifice that opens to the classification zone. In the classifying chamber, the powder is separated into a coarse powder fraction, a median powder fraction and a fine powder fraction by the action of centrifugal force produced by the curved gas streams flowing along a Coanda block 26, and classified into the respective fractions through means of classifying edges 117 and 118 each having a tapered end.
In such a conventional classifier 101, however, the pulverized feed material (feed powder) is fed through the feed supply nozzle 16, where the feed powder that flows through the inside of a convergent pipe has a tendency to flow with a driving force straight-forward in parallel with the pipe wall. In the feed supply nozzle 16, the feed powder, when fed from its upper part, is roughly separated into an upper stream and a lower stream. In the upper stream, light fine powder tends to be contained in a larger quantity and, in the lower stream, heavy coarse powder tends to be contained in a larger quantity. Since particles of the respective powders flow independently of each other, they form loci which are different in dependence on the portions at which they are fed into the classifying chamber, and the coarse powder disturbs the locus of the fine powder in the upper-part stream. Hence, it is difficult to further improve classification precision, so that the classification precision may be lowered when a powder having a large quantity of coarse particles with particle diameters of 20 .mu.m or larger is classified.
As binder resins used in toners, it is common to use resins having a low melting point, a low softening point and a low glass transition point. When a powder containing such resin is introduced into the classification zone to carry out classification, the particles may be adhered or melt-adhered to the inside of the classifier.
In recent years, as measures for energy saving in copying machines, it has become popular to use soft materials such as wax as binder resins so that toner is fixed to recording mediums such as transfer mediums by pressure, to make fixing speed higher even in the case of heat fixing, and to use binder resins with a low glass transition point or binder resins with a low softening point so that power consumption necessary for fixing can be decreased and fixing can be carried out at a low temperature.
In addition, in order to improve image quality in copying machines and printers, toner particles are made gradually finer and finer. In general, the finer the substances, the larger the force acting between particles. The same applies also to resin particles and toner particles, and the particles are more liable to agglomerate as their particle size is smaller.
Once an external force such as impact force or frictional force acts on agglomerates of such particles, the particles may be fusion bonded to the vicinities of a feed powder intake and a high-pressure air intake in the case of a material feed system shown in FIG. 17, and also melt-adhered to the inside of the classifier. In particular, the particles tend to adhere to the tips of classifying edges. Once such a phenomenon arises, the classification precision is deteriorated and the classifier is not operational in a stable state, so that it may be impossible to obtain good-quality classified powders over a long period of time.
From such viewpoints, it is sought to provide a gas stream classifier that can stably and efficiently classify fine resin powders such as, in particular, toners in a good precision.